A method of determining cardiac output during extracorporeal membrane oxygenation is being developed. This method will utilize the oxygen transferred by the membrane oxygenator as a marker in an indicator-dilution type scheme. This should enable essentially continuous on-line information as to patient cardiac output. Determinations with such a device will be carried out during interventions, both pharmacologic and physiologic, which alter the cardiac output of an anesthetized sheep undergoing extracorporeal vein to vein bypass. Such determinations will be compared to cardiac output data derived from conventional techniques.